As sensor elements that output an electrical signal corresponding to a radiation dose of incident radiation, in particular, X-rays, a direct conversion type that directly converts X-rays into an electrical signal and an indirect conversion type that converts X-rays into light by using a scintillator and then the light is converted into an electrical signal by using a photoelectric conversion element have been used. X-ray image capturing panels in which such sensor element is provided for each pixel of a plurality of pixels disposed in two-dimensional matrix form on a substrate (a panel) have been developed. In such panels, thin film transistor elements (TFT elements) are used for the control of each pixel. Further, in both the direct conversion type and the indirect conversion type, an electrical signal (an electric charge) generated corresponding to the radiation dose of X-rays is accumulated in a capacitance inside each pixel.
A detector that transfers electrical signals accumulated in the capacitances to an amplifier, which is outside the panel, via the TFT elements is referred to as a detector of a passive pixel type, and is already put into practical use in a broad range of digital X-ray image capturing devices.
Meanwhile, as described in PTL 1, NPL 1, and NPL 2, a detector referred to as an active pixel type that amplifies, by using the TFT elements as amplification elements, capacitance in which electrical signals are accumulated and transmits the capacitance to an external circuit has been developed, because it is possible to mitigate effects of thermal noise of reading lines and noise of external reading circuits.
FIG. 9 is a view that illustrates an example of an active pixel 101 and a reading circuit 102 provided in an active pixel type radiation detector 100 of the related art.
As illustrated, Vs_b, which is a bias voltage of a sensor element 103, is applied to an end of the sensor element 103 in the active pixel 101. Further, when X-rays are incident on the active pixel 101, an electrical signal is generated by the sensor element 103, and the voltage of a gate electrode of an amplifying transistor 105 connected to the sensor element 103 changes. This is because the generated electrical signal is accumulated in an electrostatic capacitance connected to the gate electrode of the amplifying transistor 105.
The amplifying transistor 105 outputs the change in the gate voltage that arises due to the generated electrical signal as a current change between the drain and the source. The amplifying transistor 105 is a transistor that amplifies the electrical signal, and the power source voltage thereof is Vd.
A reset transistor 104 controls an electrical connection state or an interruption state of the gate electrode of the amplifying transistor 105 and a reset voltage Vd applied from outside the active pixel 101 on the basis of a reset signal supplied via a reset signal line 109.
A reading transistor 106 is a switch for outputting a drain-source current of the amplifying transistor 105 to outside the active pixel 101, and is controlled on the basis of a reading signal supplied via a reading signal line 108.
Further, the drain-source current of the amplifying transistor 105, which is output to outside the active pixel 101, is read by the reading circuit 102, which is provided with an integration amplifier 107, and a capacitance Cf110 connected between a minus input terminal of the integration amplifier 107 and an output terminal of the integration amplifier 107, and the reading circuit 102 outputs an output voltage Vo111.